Anti-platelet factor 4 (PF4)/heparin antibodies (Abs) and activation of platelet FcyRlla are involved in the pathogenesis of heparin-induced thrombocytopenia (HIT). However, the actual basis of the observed thrombocytopenia and thrombosis are not well understood, and markers of risk or diagnosis and targeted therapies remain limited. This Project focuses on furthering our understanding of the cellular events underiying the thrombocytopenia and thrombosis in HIT. We believe that platelets, leukocytes and endothelium, and derived microparticles (MP) contribute to both the thrombocytopenia and thrombosis. In this Project, we propose three novel interrelated cellular mechanisms that underiie the thrombocytopenia and prothrombotic nature of HIT: